In recent years, there have been developed optical recording media (concurrent ROM-RAM optical disks; hereinafter will be simply called ROM-RAM optical disks) having a laminated structure with a laminating ROM layer from which data having been recorded beforehand can be reproduced (read out) and with a laminating RAM layer to/from which data can be recorded/reproduced (read out) In such ROM-RAM optical disks, data reproduction from the ROM layer and data recording to the RAM layer are concurrently performed, or data reproduction can be concurrently performed from the ROM layer and from the RAM layer (see, for example, the following patent document 1.
Here, the ROM layer is a part which functions as a so-called ROM (Read Only Memory), such as a compact disk, on whose surface phase pits are formed as pits and projections, thereby recoding data (ROM data) which is only reproducible. The depth of phase pits recorded as ROM data is set so that the optical intensity modulation at the time of reproduction is the maximum.
The RAM layer is a part which functions as a so-called RAM (Random Access Memory), such as an MO (Magneto Optical), to which data (magneto-optical signals, RAM data) is recorded by use of magnetism and laser, and from which data is reproduced by use of laser. When magneto-optical signals are read out from the RAM layer, weak laser light is emitted onto the RAM layer, whereby the polarization surface of laser light (RAM layer) is changed according to the direction of magnetization of the recording layer (RAM layer) due to the polar Kerr effect. Signals are detected based on the polarization direction of light reflected from the RAM layer, whereby RAM information (magneto-optical signal) is read out.
FIG. 15 through FIG. 17 show a ROM-RAM optical disk. FIG. 15 is a schematic plan view of a ROM-RAM optical disk; FIG. 16 is a schematic diagram showing a section taken in the radial direction of the ROM-RAM optical disk of FIG. 15; FIG. 17 is a schematic plan view of an important part of the same disk, which part is enlarged in illustration for description of the recording state of the ROM-RAM optical disk of FIG. 15.
As shown in FIG. 15, like general compact disks, a ROM-RAM optical disc is provided with a read-in 1 which guides the start of reproduction or recording of data and with a read-out 2 which guides the end of reproduction or recording of data. Between the read-in and the read-out area, a user area 3 in which data is to be recorded is provided. As shown in FIG. 16, the user area 3 has a laminated structure with a substrate 10 made of, e.g., polycarbonate, a dielectric film 11, an magneto-optical recording film 12 made of, e.g., TbFeCo (terbium iron cobalt), a dielectric film 13, an Al (aluminum) film 14, and a UV (Ultraviolet) hard film 15 as a protection layer.
As shown in FIG. 16 and FIG. 17, in such a ROM-RAM optical disk, ROM data is recoded in a fixed manner as phase pits 16 (double hatched areas in FIG. 17) which are formed on the substrate 10 in the form of pits and projections. RAM data is recorded as phase pit string magneto-optical records (magnetic marks) 17 (shaded areas in FIG. 17) in the magneto-optical record film 12.
The sectional view of the ROM-RAM optical disk of FIG. 16 is taken along line X-X′ of FIG. 17. In the ROM-RAM optical disk of FIG. 17, phase pits 16 serve as tracking guides, and thus no grooves as tracking guides are formed thereon.
Next, referring to FIG. 18 and FIG. 19, a description will be made hereinbelow of a common recording/reproducing method for the above-described ROM-RAM optical disk. FIG. 18 and FIG. 19 are diagrams each showing a relationship among the pulse timing of laser light, a magnetic field direction, and RAM record marks.
In the above-described ROM-RAM optical disk, in order to reproduce phase pits 16 (that is, ROM record marks) on the substrate 10 of the ROM layer while recording magnetic marks 17 (that is, RAM record marks) in the RAM layer, it is conceivable that DC (Direct Current) continuous laser light be emitted as shown in FIG. 17. In this manner, with DC continuous laser light being emitted, the direction of a magnetic field is switched, thereby forming a magnetic mark 17 with an arbitrary length. In addition, with such DC continuous light, it is possible to reproduce ROM data.
Here, to record a greater amount of data by recording such data densely in an optical disk has been a long-term problem. In the above ROM-RAM optical disk, also, dense recording of magnetic field modulation records is desired. As a resolution of the problem, there is such a method as is shown in FIG. 19, in which emitted laser light is pulsed once per minimum record unit of a magnetic mark, which is a RAM record mark. That is, the laser light is pulsed once per record clock cycle (such a pulse will be hereinafter called “once pulse”). In this manner, when laser light is emitted with once pulse, jitter at the time of reproduction of a magnetic mark 17 (RAM data) is improved in comparison with a case in which DC continuous laser light is emitted to reproduce data in the RAM layer.
However, in order to reproduce ROM data in synchronism with recording of magnetic marks 17, the method in which laser light is emitted with once pulse is not preferable. That is, due to pulse modulation accompanying pulse emission of laser light, the intensity of the laser light is varied, which will cause noise when ROM data is reproduced, so that jitter is deteriorated.
In this manner, improving jitter at the time of reproducing data in the RAM layer while densely recording data in the RAM layer, has a trade-off relationship with suppressing reproduction jitter of the ROM layer, and thus the compatibility therebetween was difficult to obtain.
With the foregoing problems in view, it is an object of the present invention to provide a recording method for an optical recording medium which realizes improvement in reproduction jitter of data recorded with laser light in the RAM layer and also improvement in reproduction jitter of data in the ROM layer at the same time.
Patent Document 1
Japanese Patent Application Laid-open No. HEI 6-202820